I. Field of the Invention
This disclosure relates to the use of anti-fungal peptides as defense against plant pathogens such as Phytophthora, Phakapsora, and Uromyces. This disclosure also relates to transgenic plants expressing such anti-fungal peptides.
II. Description of the Related Art
Invasive plant pathogens present a constant challenge to the agricultural economy. The impact of invasive pathogens in terms of crop loss and management costs is estimated at $23.5 billion annually in the U.S. (26). Among the most significant invasive pathogens are fungi and oomycetes. Invasive fungal species are difficult to detect because they may be hiding in plant tissues that are distributed or commercially exchanged. See Rossman (30). The extreme diversity of fungi and the lack of comprehensive taxonomic description also make it difficult to rapidly identify, diagnose, and mitigate fungal infection.
Phakopsora pachyrhizi is a fungus that may cause the rust disease of soybean (Glycine max), also known as Asian Soybean Rust. This pathogen has spread from Asia to all other major soybean production regions in the world. P. pachyrhizi arrived in the U.S. during the fall of 2004. At present, there is no known durable resistance available in any soybean varieties. Uromyces appendiculatus is a fungus that causes rust on bean (Phaseolus vulgaris), among others.
As rust-inducing fungi, U. appendiculatus and P. pachyrhizi belong to the order Uredinales, within the class Basidiomycetes. U. appendiculatus produces five spore stages on a single host plant. P. pachyrhizi reproduces predominantly by uredospores on a single host plant. Uredospores are responsible for rapid spread of the fungus. P. pachyrhizi can infect dozens of legume species, in addition to soybean.
Uredospores of U. appendiculatus typically penetrate through foliar stomatal openings, while germinated uredospores of P. pachyrhizi usually penetrate directly through the leaf epidermal cell layer. A urediniospore that lands on the surface of a soybean leaf germinates to produce a germ tube that, in turn, produces an appressorium and penetration hypha. As is typical for biotrophic pathogens, further colonization of tissues occurs within the apoplast, with haustorium formation occurring intracellularly (5, 12, 13).
P. pachyrhizi can infect almost any cultivar that have been tested so far. No durable, natural resistance to rust has been found in more than 18,000 soybean varieties. Breeders have been trying to identify genes in soybean or bean that can be manipulated to confer rust resistance. Despite extensive research, no durable single-gene resistance to Asian soybean rust has been discovered (8).